The present invention generally relates to firefighting equipment, and more specifically, to compressed air foam systems used to mix a stream of water with foam chemical and compressed air to produce a water/foam/air mixture for firefighting purposes. Even more specifically, the present invention relates to systems for controlling the introduction of air into the water and foam chemical mixture ratiometrically.
The addition of foaming agents to firefighting water streams is known and can be particularly useful for fighting fires, for example, fires in industrial factories, chemical plants, petrochemical plants and petroleum refineries. The use of compressed air firefighting foam requires that air and a foam concentrate be mixed and added at constant proportions to the water stream. When the foam extinguisher solution is delivered, the foam effectively extinguishes the flames of chemical and petroleum fires as well as Class A materials which would otherwise not be effectively extinguished by the application of water alone.
Foam supply systems are known in the art by the term CAFS (Compressed Air Foam System) and WEPS (Water Expansion Pumping System). A typical system includes a foam injector system, a water pumping system, and an air system including an air compressor for supplying air under pressure. For example, when employing mixture ratios of 1 CFM of air to 1 GPM of water, these systems can produce very desirable results in fire fighting by the use of “Class A” or “Class B” foams to help achieve fire suppression and to deal with increased fire loads and related hazards.
Control of the foam concentrate addition to the water stream in the appropriate proportion is significant. If an excessive amount of foam concentrate is added, a lower fire-extinguishing quality can result due to an increased foam viscosity which limits the flowability of the foam and the ability of the foam to be spread on the fire. Further, the addition of excessive amounts of foam concentrate to the water stream increases the cost of the use of the foam and the frequency at which the foam concentrate supply must be replenished at the scene. With Class A foam, surface tension reduction is optimum at a specific injection ratio; too much or too little foam chemical will lead to increased surface tension which limits water absorption into Class A or woody, cellulose type fuels. Thus, it is important to fire fighting efficiencies to maintain proper control of the foam injection rate.
The amount of air added to the water and foam chemical mixture must also be properly regulated and controlled in the appropriate proportion. Controlling the amount of air introduced into the water and foam chemical mixture is necessary to achieve the desired consistency of foam. Firefighting foam that is either too watery due to insufficient air or too dry due to excessive air is less effective at fighting fires. Dry foam made by adding extra air to the foam solution has value in exposure protection and sealing the vapors on liquid spills; however, it is not effective for direct fire attack because there is not enough water content in the foam to cool the fuels.
As the nozzle operated by the firefighter at the end of the hose line is closed, extra air or water will tend to flow into the hose line depending on which one has a higher pressure. This may contribute to an unbalanced foam mixture. Existing firefighting foam systems have had difficulties in maintaining the pressures of the water and air equal to each other. The condition in which an excessive amount of air is introduced with the nozzle closed to create the foam is commonly referred to as air packing or just packing of the hose. Some firefighting foam systems recognized this and proportion the air introduced into the water using a venturi device. However, existing air proportioned systems generally increase the size, weight and cost of the firefighting foam system. Other firefighting foam systems use an operator to control the introduction of air by constantly making manual adjustments to maintain a desired foam mixture. Changes in hose elevation, length, nozzle opening and nozzle type can require the operator to compensate with manual adjustments.
In addition to controlling the introduction of air into the water and foam chemical stream to achieve a desired foam consistency, it is also desirable to reduce the air flow or completely shut off the air flow under certain conditions. For example, if foam chemical is not being added to the water then air should stop being introduced into the water stream. Air and water do not mix under pressure. If air is added to the water without the foam chemical the unmixed air and water will cause violent surging of the firefighting hoses, commonly called slug flow. The violent surging action can be sufficiently forceful to knockdown or injure the firefighter who is operating the fire hose.
When using the prior art systems without automatic controls, it is difficult under fire fighting conditions to maintain the water pressure and the air pressure at desired levels. At a fire fighting scene, unless an operator is present at all times to observe the flow conditions and is skilled at operating the equipment to make the necessary adjustments thereof, it is possible for the system to run out of water, to run out of foam, to lose prime in the water pump, to mix air with water by itself without the foam concentrate, to put air into the system by itself, and to even overpressurize the air. The occurrence of any of the above events, in addition to the occurrence of other possible problems, can be hazardous to the firefighter.
Some CAFS that adequately control the air/foam and water/foam ratios are disclosed in U.S. Pat. Nos. 5,255,747 of Teske et al. and 5,411,100 of Laskaris et al., which are incorporated by reference herein. The system of U.S. Pat. No. 5,411,100, in particular, discloses an automatically controlled CAFS which automatically controls compressed air flow.
However, what is needed but not provided by the prior art is an improved compressed air foam system which automatically controls the air flow into the mixture. Further, what is needed but not provided by the prior art is an improved compressed air foam system which automatically controls the ratio of air to foam into the mixture to optimize the resultant mixed output. Even further, what is needed but not provided by the prior art is a compressed air foam system which automatically controls the water flow to achieve higher air concentrations than otherwise possible.